Getting bitsize of a real kind type

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1

Is there a way to determine the number of bits/bytes of a real kind value representation? The bit_size intrinsic works on integer types only and the storage_size may be (and really gets) affected by the memory alignment requirements. Its definition says that it returns the size occupied in memory by a single array element of the type of its argument. Which, e.g., for 10-bytes reals available in x86_64 architecture, gives 128 bits, not 80.

3

The answer depends on what you intend to use that information for. Do you want to know how much memory a thing takes up? If so you do want storage_size. If you want to know how much precision a number can have, use the inquiry functions like precision, maxexponent, etc.

4

I would rather want to get precision, ie. number of bits carrying information, not memory occupation. I know the inquiry functions that you have mentioned but I guess there is no such function which would give number of bits for the exponent and mantissa and/or whole number. The inquiry functions return information in rather human-readable format (as max decimal exponent, number of decimal (!) significant digits etc.)

5

DIGITS(1.0) gives you the number of significant digits in REAL (p in the real number model of Fortran), which is the number of significant bits if RADIX(1.0) is 2 (which is the case for the binary IEEE modes). Nothing equivalent for the exponent but you have MINEXPONENT(1.0) and MAXEXPONENT(1.0).

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6

Fortran is a high level programming language, so it can be difficult to obtain answers to low level details such as the number of bits in the mantissa of a real, etc. If you will be content with code that will work on machines with a known real number representation, such as IEEE 32-bit reals on little endian machines, consider the following code:

program sigbit
! locate least significant exponent bit of little endian IEEE real.
implicit none
integer i
real x, y
!
x = 1.0
y = 2.0
do i=0, 30
   if(btest(transfer(x,i), i) .neqv. btest(transfer(y,i),i))then
      print *,'Mantissa has ',i+1,' bits'
      print *,'Exponent has ',31-i,' bits'
      exit
   endif
end do
end program

On the other hand, it is easier to remember 1, 8, (1)+23 as (sign), (biased exponent), (implicit integer part of mantissa), (fractional part of mantissa)

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7

DIGITS() - that one I have forgotten. Thanks @themos